There are many known processes for producing polycrystalline silicons used as semiconductors and photovoltaic cell materials, and some processes are performed in the industry.
One of such processes is the so-called Siemens process, in which a silicon rod heated by energization to a silicon deposition temperature is placed in a bell jar, and trichlorosilane (SiHCl3, hereinafter TCS) or monosilane (SiH4) together with a reducing gas such as hydrogen are brought into contact with the rod to deposit silicon.
This process provides high-purity silicon and is performed most commonly. Because of batchwise deposition, however, the process has a problem of a very complicated procedure including placement of the silicon rod as a seedbed, energization heating, deposition, cooling and takeout of the silicon rod, as well as bell jar washing.
To solve the above problem, the present applicant has proposed a silicon production reactor capable of producing silicon continuously and stably over extended periods (Patent Document 1, JP-A-2002-29726). The reactor is structured such that a silicon deposition feedstock gas is supplied into a tubular reaction vessel resistant to temperatures in excess of the melting point of silicon, the tubular reaction vessel is heated to deposit silicon, and the deposited silicon is molten and continually drips down from the lower end of the tubular reaction vessel and is recovered.
This reactor is very advantageous in that the conventional problems with the Siemens process are solved and silicon is produced continually. However, it has been revealed that the tubular reaction vessels disclosed in Examples of Patent Document 1 that have a simple internal structure circular or polygonal in cross section cause a lowered reaction rate of the feedstock gas when the vessels are scaled up without any modification for industrial-scale production of more than several hundreds of tons of silicon annually.
Furthermore, the scale-up tends to increase the probability of generation of by-products such as silicon fine powder and silane oligomers, resulting in lower silicon yields. Moreover, the by-products often adhere to a reaction gas discharge line to cause blockage. Therefore, improvements of these problems have been desired.
Filling a reaction vessel with a filler or the like is known as means for increasing the reaction efficiency of the feedstock gas (Patent Document 2, JP-A-S59-162117).
However, the following problem is often encountered. The silicon deposition reaction vessel is generally heated inside by heat conduction from external heating means, and therefore heat cannot reach deep into the filler layer. As a result, a great temperature difference is caused within the filler layer between the vicinity of the reaction vessel wall and the vicinity of the filler layer central axis.
Sufficient heating to the vicinity of the central axis is particularly difficult with a scaled-up large diameter reaction vessel, even with the use of heating means such as a high-frequency induction heating system or a dielectric heating system which relatively facilitates deep heating, and then, ultimately, a solid deposit clogs the filler layer. When the heating output is increased to solve the above problem, a vicinity of the heating means, for example the external wall of the reaction vessel, is heated to an extremely high temperature, so that the reaction vessel material is remarkably deteriorated, leading to a new problem such as difficult long-term operation.
Patent Document 1: JP-A-2002-29726
Patent Document 2: JP-A-S59-162117